The present study is a continuation into adulthood of a major longitudinal twin study of aggression and antisocial behavior (ASB) and its social and biological risk factors (#MH58354) in a diverse urban sample. The first two funding periods provided comprehensive laboratory assessments of behavioral, neurocognitive, social and psychophysiological function, and multi-trait, multi-rater assessments of externalizing psychopathology during four waves of investigation when the twins were ages 9-10 (Wave 1) 11-13 (Wave 2), 14-16 (Wave 3) and 17-18 years old (Wave 4). Relationships of ASB with known risk factors (social and biological) were confirmed, and new relationships identified. The extent to which genetic and environmental factors affect ASB, its risk factors and their relationships has also been elucidated, with many findings shown to vary across gender, informant, definition of ASB, and age. We propose to extend the study to include an adult assessment at ages 19-23 (Wave 5), to obtain comprehensive measures of antisocial and aggressive behavior and their risk factors during a period of greatest risk for criminal offending, antisocial personality disorder and substance abuse. The primary aim of the next phase is to understand environmental, genetic, and phenotypic continuities in externalizing behavior problems and their biological and social risk factors through adulthood, including the prediction of adult outcomes from childhood and adolescent measures. Adding the adult assessment will yield the most comprehensive, prospective longitudinal data on externalizing behavior problems ever obtained in a genetically informative design. This five-wave, 15 year longitudinal twin study will provide the unique opportunity to understand how genes and environment combine and interact to produce antisocial outcomes from childhood to adulthood, and will greatly enhance our understanding of externalizing psychopathology and its heterogeneous developmental trajectories. Understanding gene-environment interplay in the development of externalizing psychopathology is of key importance to future intervention and prevention.